In a conventional radio transmission system, a fading characteristic of a channel is one of main causes of a bit error. In a multiple input multiple output (Multiple Input Multiple Output, “MIMO” for short) technology, a transmit diversity gain may be obtained by using a space time coding technology, so as to expand a channel capacity in a space domain. A spatial multiplexing technology of MIMO includes single user MIMO (Single User MIMO, “SU-MIMO” for short) multiplexing and multi-user MIMO (Multiple User MIMO, “MU-MIMO” for short) multiplexing, where SU-MIMO means that multiple parallel data streams are transmitted between one sending station (Station, “STA” for short) and one receiving station by using multiple antennas. MU-MIMO means that multiple parallel data streams are simultaneously and separately sent between one sending station and multiple receiving stations or between multiple sending stations and one receiving station by using multiple antennas. Currently, an SU-MIMO technology and a downlink MU-MIMO technology are introduced to the Institute of Electrical and Electronics Engineers (Institute of Electrical and Electronics Engineers, “IEEE” for short) 802.11 protocol, and it is a development trend in the future to introduce an uplink MU-MIMO technology to the IEEE 802.11 protocol. In the uplink MU-MIMO technology, multiple stations simultaneously and separately send uplink data to an access point (Access Point, “AP” for short) by using multiple antennas. Correspondingly, the AP separately demodulates the data of the stations by using multiple antennas, to improve space utilization and improve a system throughput.
In an existing uplink MU-MIMO technology, after obtaining a transmission resource through contention, a station sends a Request To Send (Request To Send, “RTS” for short) frame to an access point (Access Point, “AP” for short), and the AP measures a transmission channel between the station and the AP according to the RTS frame of the station, and continues to receive an RTS frame sent by another station until a quantity of received RTS frames is equal to a preset value. Subsequently, the AP replies a Clear To Send (Clear To Send, “CTS” for short) frame to all stations that send RTS frames, to instruct these stations to send uplink data, and separately adds, to the CTS frame sent to all the stations, channel state information (Channel State Information, “CSI” for short) of channels that are measured by the AP and between the AP and the stations. The stations that receive the CTS frame sent by the AP may determine transmission rates according to the CSI indicated in the CTS frame, and simultaneously send uplink data to the AP according to the transmission rates. In the foregoing uplink MU-MIMO technology, each station needs to contend with another station for a transmission resource, and can send an RTS frame to the AP only after obtaining the transmission resource through contention, and network overheads are relatively high. Further, the AP instructs the station to send uplink data only after receiving a preset quantity of RTS frames, which may cause a phenomenon in which the station waits for a relatively long time because the AP cannot receive enough RTS frames in a relatively long time. Consequently, a transmission delay is relatively long, user experience is relatively poor, and time-frequency resources of a system are wasted.